Proteoglycans are specialized glycoproteins that usually carry glycosaminoglycans (GAGs). GAGs are linear polysaccharides with alternating regular sugars and amino sugars. These molecules are especially important in cell signaling due to their unique location on the cell membrane and in the extracellular matrix. They also have the ability to interact with and modulate proteins found in extracellular matrix such as growth factors and cytokines. Proteoglycans can be grouped according to the types of GAG they carry or by the degree of homology between their core proteins. GAGs include heparan sulfate, chondroitin sulfate, keratin sulfate, and dermatan sulfate. A single proteoglycan can carry one or more types of GAGs.